To improve metal strength, a workpiece such as a steel workpiece is hardened by performing an induction hardening. The induction hardening involves a quenching of a workpiece, forming a hardened layer at the surface of the workpiece. Mechanical characteristics varies depending on the depth of the hardened layer. Thus, quenching is performed under preset conditions, and a quality check is conducted after the production.
Conventionally, a workpiece is randomly selected and cut to check whether a quenching is properly being performed. However, with this method, not only the checking work takes time but also the workpiece selected for the checking work cannot later be used as a product.
According to a related art, an apparatus is provided to measure a depth of a hardened surface layer of a workpiece. The apparatus includes a magnetizer to magnetize the workpiece, a detecting coil to detect an induction magnetic field generated by the magnetizer, and quenching depth determining means for deriving the quenching depth of the workpiece from the output voltage value of the detecting coil and already-known information on magnetic characteristics of a material equivalent to the workpiece (see, e.g., WO2012/057224A1). The magnetizer has a U-shaped yoke, an excitation coil wound on a portion of the yoke that faces the workpiece, and a detection coil provided at a lower portion of a leg of the yoke.
In this related art example, the yoke has pair of leg portions and a base portion coupling the leg portions t each other, and is arranged such that the open side of the U-shape faces the workpiece. When current is applied to the excitation coil provided on the base portion, the magnetic flux flows through one of the leg portion, the workpiece, the other leg portion and the base portion in this order, and with this magnetic flux, a spatial magnetic flux is generated on the surface layer of the workpiece. The detecting coil detects this spatial magnetic flux. However, because the detecting coil is provided on the leg portion of the yoke, not only the spatial magnetic flux but also the magnetic flux generated inside the yoke are detected, so that measurement accuracy may not be sufficient.